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General tips about sanding and drywall compound...

When repairing drywall, your goal is to do as little sanding as possible.  It's almost impossible to make a repair without at least a tiny amount of sanding.  The best pros hardly sand at all, but this ability comes with years of experience and lots of sanding along the way!  That's one reason why the pros work so quickly... they are precise and know their products intimately 

 Sanding drywall compound produces a fine dust that gets onto and into everything.  Because it is water-soluble, wiping the dust with a sponge makes a royal mess!  By collecting the dust at the work area, either through wet sanding or through use of a vacuum, you eliminate much of your cleanup.  Since small holes generate small amounts of dust, these simple methods are more than adequate.

The best way to minimize sanding is:

  • Never apply the drywall compound in thick layers because it can't be made smooth when thick and tends to crack while drying.
  • Don't overwork the compound (because it dries quickly and loses its creaminess).  Apply it, make a few smoothing stroke and then WALK AWAY!
  • If the compound seems too thick, it probably is.  Add a few drops of water... it doesn't take much to make the compound nice and creamy. 
  • NEVER REUSE OLD COMPOUND!!  Putting old compound back in the can will make the mix lumpy.  Throw away any compound that has been removed from the can, even if never touched!
  • Buy a new can for a new job... especially IF there is any dried compound in the old can or it has dark streaks in it (most likely mildew).  Again, those hard lumps will mess up your attempts to create a smooth finish. 
  • Use a dust mask whenever sanding drywall compound.  Inhaling too much of this fine dust can possibly damage your lungs.

Between coats, use your drywall knife to shave off any lumps instead of sanding them.  If you've followed these tips you should need minimal sanding.

Back to hole repair.  First things first... look at the hole!

Know thy drywall!  Drywall is made from a special durable paper that is pressed onto a pressure-formed layer of compressed rock dust, a.k.a. gypsum.  The gypsum gives the wall its hardness and the paper protects the gypsum from moisture while providing a smooth surface for painting while adding some flexible strength.

The smaller and "neater" the hole, the easier it is to fix.  Here is a rough guide:

  1. Holes up to about 1/2" can be filled with most wall patching compounds, such as drywall compound or light duty spackle.
  2. Holes between 1/2" and 6" can be repaired with a piece of drywall and drywall compound. 
  3. Holes larger than 6" can be repaired with a piece of drywall and drywall compound but should have reinforcing wood strips installed in the wall to support the drywall edges.
  4. Since wall studs are a maximum of 14 1/2" apart, very large repairs will bridge across one or more studs.  We'll talk about large repairs in another article.

Repair rules for very small holes...

Very small holes in drywall... from nails, screws, picture hangers, wall anchors...  don't need much preparation aside from cleaning away any dust.

  • Use a hammer to gently tap the wall to cause a slight depression.  You can also use the rounded end of a screwdriver (or other tool) and tap the other end with a hammer for more accuracy.  Filling this depression will give you the most invisible patch since you don't have to "feather" the repair to make it appear flat.  You can sometimes finish the repair with a single coat followed by light sanding.
  • If the paper around the hole is loose or ragged, you can carefully cut out the paper, though on very small holes this is rarely needed.
  • SECRET TIP TO MAKE OLD BOLTS DISAPPEAR:  If you have a  bolt in the wall, you'll find them difficult to remove without making a huge hole.  So don't try to remove the molly.  Instead, use your handy hammer and a screwdriver and tap it lightly till the metal collar is slightly depressed below the level of the wall.  Cover the entire head with a layer of drywall compound and voila... all gone!

Repairs for medium sized holes - up to about 6" across

There are two ways to repair these holes. (There are more but I want to get home for dinner.) I prefer the first but some people like the second.  The second is definitely easier, though!

Method 1: Plasterboard patch screwed to supporting braces

  • If the paper around the hole is torn, loose or ragged, cut back the loose paper with a very sharp utility knife.  Squaring up the hole can make the repair easier, but it is not absolutely necessary.  At the least, cut off any hanging pieces of drywall.  (For example, if you accidentally smashed your wall through with a hammer, you'd know what I mean!  Ouch.)
  • Install a wood backer support inside the wall behind the hole.  This can be made from any piece of scrap wood you have lying around... even a paint stirrer will work.  I prefer to use scrap pine because it is easy to cut and easy to screw into.  Cut the backer as wide as possible so it will insert into the hole but be long enough to extend at least a few inches beyond the edges of the hole.  Up to 3" you only need a single backer.  For larger holes, use two, one near each opposite edge, to add more rigidity.
  • Cut a piece of drywall slightly smaller than the hole so it fits easily into the hole.
  • Put the scrap drywall in place and screw it to the backer(s).  You can also glue it in place with hot melt glue, or use screws plus construction adhesive.  Whatever floats your boat.
  • Taping is not necessary for holes up to 6", though this is a matter of personal preference.  I prefer to simply force drywall compound into all the cracks, working it deeply so it pushes through the cracks.  The excess compound that pushes into the wall hollow, when it hardens, will "key" the repair and make the seams firm and crack-free.  Allow at least 24 hours drying time and do not disturb the repair during that time.  You might notice some cracking as the compound dries.  This is normal for thick coats of drywall compound and will be covered completely by subsequent coats.
  • If you want the job to move faster, you can substitute Plaster of Paris for drywall compound for this first fill.  The plaster dries extremely hard and, because it sets in less than a half hour, you can do the second coat in one day.  Be sure than the Plaster is not level with the surface for this first fill... push it strongly into the cracks but leave it at least an 1/8" below the wall level and rough so the drywall compound can stick firmly to it.  IF YOU LEVEL IT UP, THE PLASTER WILL EXPAND WHEN DRYING AND IT WILL RAISE ABOVE THE WALL LEVEL.  Plaster of Paris is very hard to sand and you'll end up with a mess!!
  • Expect to apply one or two coats of drywall compound after the initial fill.  The larger the hole, the further you should feather out the compound.  If you apply the compound too thinly, you will see the paper edges of your cut.  You should apply the compound just thickly enough to visibly cover the repair and STOP.  Do no try to work and work and work the compound.  It will only begin to dry and get more ornery!  Walk away, let it dry and come back for the second coat later.
  • When the first coat is dry, use your drywall knife to knock off any lumps and to scratch off any high areas.  If you've handled the knife skillfully, your surface should be fairly smooth with minor bubbling or smearing.  (This is where experience is helpful.) Use the second coat to thinly fill in these defects and feather the repair out a little further.  Again, do not overwork the compound!

Method 2:  Plasterboard trimmed and glued directly to existing wall

I said earlier that I prefer Method 1, but frankly Method 2 is so much easier.  I guess it's hard to teach old handymen new tricky techniques!!

  • Square up the hole using a utility knife or a drywall cutting saw.
  • With a pencil, draw a line outside the perimeter of the hole, about an inch outside the edge. With a utility knife, cut through just the paper along this line.  Peel the paper off.
  • Using the drawn line, make a drywall patch the same size. 
  • On the back of the drywall, mark the profile of the hole, centered within the drywall patch. Choose any side.  Along one line, cut through the back of the drywall just enough to cut the paper and score the drywall.  Bend the drywall sharply towards the front to break off the drywall along the score.  Carefully peel the gypsum core off the paper, leaving the paper.  Do this on all sides.
  • Test fit the patch.  The gypsum should fit into the hole and the overhanging paper should fit within the trimmed paper on the wall.  If not, trim either the gypsum or the paper till you have a slightly loose fit.
  • Apply a generous amount of drywall compound to the wall where the paper has been cut.  Press the patch in place and, using a drywall knife, run the blade around all sides till most of the drywall compound has been pressed out.  The patch should now fit in the hole and the paper should be fairly level with the existing wall.  Allow to dry for at least 24 hours. 
  • IMPORTANT:  If the patch seems to be raised above the level of the wall, you can level it by screwing a small (1x2) board across the patch to push it flat.  Leave the board in place till the compound dries... at least 12 hours.  The small screw holes in the wall can be filled when you second-coat the patch.
  • Expect to apply one or two additional coats of drywall compound, feathering each out to blend with the wall surface.  Sand after last coat is thoroughly dry.

Back to dusty, messy sanding...

You're at the point where you have a fairly level patch (you have gotten good, haven't you!) but there are some rough spots.  Now is the time to consider sanding.

First of all, it's always better to fill than sand, so if you have any small holes or "bubbles", try filling them with thinned compound first before resorting to sanding.

There are no two ways about it.  Dry sanding is dusty!  There are attachments for vacuum cleaners that use sanding screens... plastic sheets with thousands of holes that are impregnated with carbide abrasive.  The vacuum sucks the dust through the holes as you sand.  They are not 100% effective, but get virtually all of the light dust which is the most messy.  Any larger particles that miss the vacuum will just drop to the floor for cleanup.

If you don't want to purchase one of these special sanding kits, you can simply hold the vacuum cleaner nozzle under the repair as you sand, which will catch much of the dust.

Wet sanding has its fans...

I'm not too big on wet sanding since it can smear the compound.  But it is the closest thing to dust-free drywall sanding there is.  If you decide to wet-sand, don't use too much water and be careful not to oversand or you'll rub off too much compound and have to recoat the repair again! (Don't feel bad... even pros have to go back and touchup their work!  At least the conscientious ones.)  There are special sandpapers that are designed to be used wet, and there are also sanding screens that work well with water.   Again, don't get the patch too wet!

When the repair is done...

No matter how large or small the repair, you should prime and paint the patch.  This protects the patch from moisture and will make it even more invisible.  I prefer to use Kilz oil primer/sealer for patches where I am only going to touchup the finish paint.  Kilz doesn't seem to cause "flashing" because it does not dry smooth.  Because it is an oil primer, it won't dissolve or soften the drywall compound.

Shellac-based primers make patches very shiny and this shininess seems to show through most finish paints, even multiple coats.  I've had mixed results with latex primers, but the one's I've used seem to be resistant to flashing.